1. Field of the Invention
This invention is in the field of drying by means of impingement flow of high pressure heated air, with provisions being made for exhausting the impinging air streams behind the impingement jets so that the impinging air streams are directed to an exhaust chamber at a relatively low velocity and without excessive pressure drop.
2. Description of the Prior Art
Impingement flow, that is, flow directed normal to the surface has been recognized as an efficient means for heating or cooling. In recent years, this method of heat transfer has been used in the paper industry for drying of paper. Representative patents in this field are U.S. Pat. Nos. 3,163,502; 3,167,408; and 3,447,247 all owned by the assignee of the present invention.
Air impingement drying is particularly suited for drying of lightweight grades of paper such as tissue paper and for drying coated paper. These applications require higher rates of heat transfer because of the limited drying length and the requirements of high speed operation.
There are various types of air impingement devices in use on paper drying apparatus. One of these types uses slotted nozzles and another incorporates round holes to provide jet orifices for impingement purposes. The slot nozzle arrangements have the disadvantage of requiring a relatively complex system of air removal ducts between the slots. Slot arrangements are also characterized by inefficient performance as measured by the heat transfer coefficient obtainable for a given expenditure of air blower horsepower. In addition, relatively small spacings between the impingement surface and the slot nozzles are required in order to obtain good heat transfer results.
Some of the disadvantages inherent in the slot nozzle arrangement are eliminated in the round hole impingement systems. For example, the heat transfer coefficient is relatively unaffected by the distance from the nozzle to the impingement surface as long as there is a proper ratio of the impingement distance to the hole diameter. Also when using round impingement holes, it becomes easier to incorporate air exhaust systems sets of the round holes.
With the demand for increased machine speeds, adequate drying must either be accomplished by raising the drying rate or the heat transfer length. Increased drying lengths require additional capital expenditure for already expensive drying equipment. In tissue drying applications, where the wet web is pressed on the surface of a large diameter rotating drum, the web must be dried in less than one revolution. Typically, such a drying system employs a large diameter steam filled cylinder surrounded by a high temperature, high velocity air impingement cap. However, these steam filled cylinders are already operating at about the highest practical steam pressures possible and are being built at about the largest practical diameter possible. Therefore, any further increases in speed must come from increased heat transfer rates from air impingement. At the present time, air caps are being operated at temperatures of about 800.degree.F. In order to achieve higher temperatures, expensive high temperature alloys must be employed. In addition, at these higher temperatures problems are encountered in maintaining the dimensional stability of the equipment and as impingement temperatures get higher, more problems will be encountered with drying uniformity.
Inasmuch as air caps in use today in the paper industry are already operating at about the limit of temperature, it becomes necessary to increase the convective heat transfer coefficient in order to increase the heat transfer rate and consequently the evaporation rate. In paper drying applications, a large convective heat transfer coefficient helps to alleviate any nonuniform drying problems. One method of increasing the convective heat transfer coefficient is simply by increasing the impingement velocity. However, for a given system configuration an increase in impingement velocity can only be obtained at the expense of increased fan horespower. Increases in fan horsepower represent both increased capital cost for equipment and also increased operating expense. Therefore, an upper limit exists whereby increases in heat transfer rate by adding additional fan horsepower are no longer considered feasible.
Another means of increasing the heat transfer coefficient is to increase the number of impinging jets, that is, by increasing the open area of the impingement plate. Published literature indicates that after the open area is increased beyond approximately 2%, no further gains in the heat transfer rate are obtainable. It was thought that the inability to improve the heat transfer rate was caused by interference between adjacent impinging jets, that is, as the open area was increased and the impingement jets became closer and closer together, it was thought that the adjacent jets interfered with each other thereby reducing the heat transfer coefficient.
More recently published experimental data indicates that this reduction in heat transfer coefficient is not caused by interference between adjacent jets but rather by cross flow interference from the spent air. The jets after impingement must travel to an opening to be exhausted and this means that the spent air must travel across adjacent jets before reaching an exhaust outlet. This exhaust cross flow interference can actually cause the impinging jet to be bent at an angle which is not perpendicular to the surface of impingement. Any deviation of the impingement jet from a line normal to the heated or cooled surface results in a degradation of the heat transfer rate. Consequently, it becomes important to eliminate or reduce cross flow interference if the average heat transfer coefficient is to be increased.